Isoprene and butadiene are normally obtained from C.sub.4 -C.sub.5 streams from petroleum refining. In addition to isoprene and butadiene, these streams contain other compounds such as acetylenes, cyclopentadienes and CS.sub.2. The acetylenes and cyclopentadienes can only be tolerated to a limited extent in the polymerization of isoprene and butadiene and must be reduced to low levels or removed. On the other hand, CS.sub.2 at the expected concentrations, does not interfere with the polymerization process. In fact, some CS.sub.2 is beneficial to the polymerization process.
However, one of the methods normally used to remove acetylenes from these streams is by selective hydrogenation. The presence of CS.sub.2 poisons the catalyst used for the hydrogenation and, thus, excessive amounts of the expensive catalysts are needed. Another method for removal of the acetylenes and cyclopentadiene is by the use of nickel octoate in conjunction with aluminum alkyls. In this process the CS.sub.2 interferes by reacting with the nickel octoate necessitating the use of excessive amounts.
This invention relates to the removal of CS.sub.2 in the purification of hydrocarbons.
After the CS.sub.2 is removed, the acetylenes and cyclopentadienes can then be readily removed by the methods previously described.
Various methods have been used to remove CS.sub.2 from solutions such as oxidations, reductions, reactions with alkaline hydroxides and reaction with alcohols in the presence of alkaline hydroxides to form xanthates. All these methods are impractical when used with C.sub.4 -C.sub.5 hydrocarbon streams, since the residual CS.sub.2 interferes with subsequent reactions required to remove the other polymerization inhibitors.
British Pat. No. 570,294 describes a method for removal of CS.sub.2 from crude benzene, forerunnings, oils and hydrocarbon vapors using amines. The separation and recovery of the amine and the CS.sub.2 follows. The process includes bringing into contact hydrocarbon as a liquid or vapor containing CS.sub.2 and a primary CS.sub.2 and a primary or secondary amine or a liquid containing such an amine in the presence of ammonia to form ammonium mono or diaryl or alkyl dithiocarbamate removing the dithiocarbamate and decomposing it to regenerate the CS.sub.2 and the amine.
British Pat. No. 617,969 describes a process for the removal of CS.sub.2 from hydrocarbons by forming a salt of a substituted dithiocarbamic acid. The CS.sub.2 is caused to react with a primary amine of a cyclic paraffin in the presence of a strong base in an aqueous solution. The solution of the salt is then separated from the hydrocarbon layer.
In both of these cases, the initial amount of CS.sub.2 is quite high (13 g/l). The residual amount of CS.sub.2 after treatment can be as high as 400 ppm and in the optimum case is reduced only to 70 ppm. The amine was used in only slightly more than molar amounts relative to the CS.sub.2. In addition, excessive amounts of alkali must be used to solubilize the dithiocarbamic acids so that they can be removed by washing with water. The residual CS.sub.2 content was excessive to the extent that additional amounts of catalyst would be used up in the subsequent catalytic hydrogenation or other treatment to remove the acetylenes and cyclopentadienes.